1. Field of the Invention
The invention relates generally to use of matrix acidizing in subterranean hydrocarbon formations. In particular aspects, the invention relates to techniques for helping to evaluate the effectiveness of matrix acidizing.
2. Description of the Related Art
Matrix acidizing is a stimulation process wherein acid is injected into a wellbore to penetrate rock pores. Matrix acidizing is a method applied for removing formation damage from pore plugging caused by mineral deposition. The acids, usually inorganic acids, such as fluoridic (HF) and or cloridic (HCl) acids, are pumped into the formation at or below the formation fracturing pressure in order to dissolve the mineral particles by chemical reactions. The acid creates high-permeability, high productivity flow channels called wormholes and bypasses the near-wellbore damage. The operation time depends on such parameters as the length of the wellbore, the rock type, the severity of the damage, acid pumping rate, downhole conditions and other factors.
Matrix acidizing is also useful for stimulating both sandstone and carbonate reservoirs. Matrix acidizing efficiency in removing the formation damage is strongly dependent on the temperature at which the acidizing occurs and weakly dependent upon the corresponding pressure. The acid temperature in the formation depends on the convective heat transfer as the acid flows through the formation and on the reaction heat transfer due to the acid-mineral reaction.
Convective heat transfer is the main mechanism for temperature change during acid flow through wormholes. The acid temperature in the wormholes may vary by as much as 10-20° C. (18-36° F.), depending on the initial temperature difference between wellbore and the formation. The acid temperature at the end of the wormholes, about 1-10 m (3.3-33 feet) from the wellbore, may increase by 1°-5° C. (1.8°-8° F.) above the formation temperature at those locations, depending on the injected acid volume.
Along a wormhole, the temperature changes over time as illustrated by FIG. 4. Initially, the temperature near the wellbore is the acid temperature inside the well (Tw at t=0). The rest of the wormhole, which may be partially or totally undeveloped, is assumed to be at the formation or reservoir temperature (Tr at t=0), which is greater than the wellbore temperature. As time progresses and acid is injected through the wormhole, at small radial distances near the wellbore (up to about 1 m (3.3 feet)), the acid temperature decreases from Tr to Tw with time at a rate depending upon the temperature drop of the fluid flowing from the wellbore. In other words, in the near well region, the temperature behavior depends only on the convection heat transfer due to the acid flow through the wormhole.
At distances further away than about 1 meter (3.3 feet) and at the advancing acid front region, the acid temperature increases from the well temperature to the formation temperature. This temperature increase is still due mainly to convection heat transfer. However, in the transition between the two temperature levels, the reaction heat transfer between the acid and minerals changes the temperature behavior by smoothing out the temperature change on one side closer to the well and by uplifting the formation temperature by about 1°-5° C. (1.8°-8° F.) on the other side, as FIG. 4 illustrates. The acid temperature changes in both regions (near well and near the acid front). It increases with time and distance due to two mechanisms. First, it depends on the time needed by the acid and minerals to react completely. Second, it depends on the contact area between acid and minerals which increases rapidly with distance. After the acid injection is stopped, the acid-mineral reactions may still continue for some time. However, these reactions take place further away from the well, where the acid front is located. Even the local temperature at the acid front may still increase after the acid injection is stopped. This temperature increase is small and cannot be recorded in the near-well region, so it can be ignored in all additional calculations. At the time when the acid injection is stopped, the temperature along the wormhole is decreasing from almost formation temperature at the wormhole end away from the well (Tr at t=ts) to the well temperature (Tw at t=ts) near the well. As time progresses, the temperature wave moves toward the well at a speed depending upon the wormhole properties (geometry, length, thermal conductivity) and formation properties (porosity, permeability, thermal conductivity, etc.). Eventually, without acid flow, the well temperature (Tw) increases until it reaches the formation temperature (Tr) at time t=tf. Thus, the total time in which the well temperature varies is tf. If the acid injection is started and stopped at times t=0 and t=ts, respectively, between 0 and ts, the well temperature decreases from Tw at t=0 to Tw at t=ts. This is illustrated by FIG. 5. Between ts and tf, the well temperature increases from Tw at t=ts to Tw at t=tf. The time in which the matrix acidizing performance can be evaluated is thus between 0 and tf or between ts and tf, depending on the evaluation technique. In addition to temperature, when the acid flow between the well (annulus) and the formation through wormholes, the local pressure drops due to the change in flow area (such as from the annulus area to the wormhole area). The pressure drop may not be relevant if there is no acid flow. Also, it is worth noting that the temperature and pressure may vary meaningfully only around wormholes (i.e., where there is radial acid flow between the well and the formation).
Methods for monitoring and evaluating matrix acid stimulations have long been investigated. Recently, distributed temperature sensing (“DTS”) technology has emerged as a tool for real-time data acquisition and interpretation for evaluating matrix acidizing performance. Although the main advantages of this technique (i.e., real time temperature data acquisition along the entire well and great sensitivity) are impressive, there are several major disadvantages as well. First, the DTS fiber is placed inside the coiled tubing string. Recording temperature data with a reasonable resolution assumes that the fiber has to stay immobile for the entire time needed for data acquisition. Second, as the DTS fiber is a multi-point temperature sensor (i.e., the fiber can record temperature data along the well at multiple locations), there is a significant amount of temperature data transmitted to the surface and being processed for all times and multiple positions along the well. Several solutions have been proposed in literature trying to circumvent these disadvantages. However, these proposed solutions are expensive and not reliable.